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The Helgeland Nappe Complex forms part of the Uppermost Allochthon in the central Scandinavian Caledonides. It consists of metasupracrustal rocks of assumed Precambrian age which are juxtaposed with metasupracrustal rocks present as non-conformable cover sequences to subjacent fragments of Early Ordovician ophiolitic rocks. All of these rock units have been intruded by the Bindal Batholith, which ranges in composition from mafic gabbro to leucogranite. In this paper, U-Pb zircon ages from five plutons in the southern part of the batholith are presented. These are a porphyritic granite of the Andalshatten pluton (447 ± 7 Ma), a tonalite from the Kråkfjellet pluton (443 ± 7 Ma), a granodiorite west of Gåsvassfjellet (437 ± 4 Ma), the Kalvvatnet monzonite (435 ± 10 Ma), and a monzodiorite near Tosbotn (430 ± 7 Ma). The dates are essentially concordant and are considered to reflect the crystallization ages of the intrusions. Based on regional correlations and the age of the oldest of the dated plutons, it is concluded that internal accretion and polyphase tectonometamorphism of the Helgeland Nappe Complex took place in Ordovician times. The final Silurian (Scandian) thrusting of the nappe stack across the continent of Baltica appears to post-date emplacement of the Bindal Batholith.

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The early magmatic and tectonic history of the Carolina terrane and its possible affinities with other Neoproterozoic circum-Atlantic arc terranes have been poorly understood, in large part because of a lack of reliable geochronological data. Precise U-Pb zircon dates for the Virgilina sequence, the oldest exposed part, constrain the timing of the earliest known stage of magmatism in the terrane and of the Virgilina orogeny. A flow-banded rhyolite sampled from a metavolcanic sequence near Chapel Hill, North Carolina, yielded a U-Pb zircon date of 632.9 +2.6/-1.9 Ma. A granitic unit of the Chapel Hill pluton, which intrudes the metavolcanic sequence, yielded a nearly identical U-Pb zircon date of 633 +2/-1.5 Ma, interpreted as its crystallization age. A felsic gneiss and a dacitic tuff from the Hyco Formation yielded U-Pb zircon dates of 619.9 +4.5/-3 Ma and 615.7 +3.7/-1.9 Ma, respectively. Diorite and granite of the Flat River complex have indistinguishable U-Pb upper-intercept dates of 613.9 +1.6/-1.5 Ma and 613.4 +2.8/-2 Ma. The Osmond biotite-granite gneiss, which intruded the Hyco Formation before the Virgilina orogeny, crystallized at 612.4 +5.2/-1.7 Ma. Granite of the Roxboro pluton, an intrusion that postdated the Virgilina orogeny, yielded a U-Pb upper intercept date of 546.5 +3.0/-2.4 Ma, interpreted as the time of its crystallization. These new dates both provide the first reliable estimates of the age of the Virgilina sequence and document that the earliest known stage of magmatism in the Carolina terrane had begun by 633 +2/-1.5 Ma and continued at least until 612.4 +5.2/-1.7 Ma, an interval of approximately 25 m.yr. Timing of the Virgilina orogeny is bracketed between 612.4 +5.2/-1.7 Ma and 586+/-10 Ma (reported age of the upper Uwharrie Formation). The U-Pb systematics of all units studied in the Virgilina sequence are simple and lack any evidence of an older xenocrystic zircon component, which would indicate the presence of a continental-type basement. This observation, together with the juvenile Nd isotopic character of the Virgilina volcanic arc sequence, suggests that the oldest part of the Carolina terrane was built on oceanic crust away from a continental crustal influence.